Add link to task when contexts documentation

Fixes #104000

src/vs/workbench/contrib/tasks/common/taskDefinitionRegistry.ts

@@ -40,7 +40,7 @@ const taskDefinitionSchema: IJSONSchema = {
 		},
 		when: {
 			type: 'string',
-			markdownDescription: nls.localize('TaskDefinition.when', 'Condition which must be true to enable this type of task. Consider using `shellExecutionSupported`, `processExecutionSupported`, and `customExecutionSupported` as appropriate for this task definition.'),
+			markdownDescription: nls.localize('TaskDefinition.when', 'Condition which must be true to enable this type of task. Consider using `shellExecutionSupported`, `processExecutionSupported`, and `customExecutionSupported` as appropriate for this task definition. See the [API documentation](https://code.visualstudio.com/api/extension-guides/task-provider#when-clause) for more information.'),
 			default: ''
 		}
 	}